There is currently ongoing work within 3rd Generation Partnership Project (3GPP) to introduce a new Radio Access Technology (RAT) called E-UTRAN, also known as Long Term Evolution (LTE). How to handle interworking between E-UTRAN and the other existing RATs, mainly GSM/EDGE Radio Access Network (GERAN) and UMTS Terrestrial Radio Access Network (UTRAN), is being specified. As part of that, so called “RAT priorities” are being defined.
The RAT priorities are settings that are to be broadcasted from the network to the user equipments in order to inform about what RAT the user equipments shall prioritize for camping selection in idle mode. An operator may thus define the general camping priorities for the user equipments. Typically, an operator would prioritize that the user equipments camp in E-UTRAN when it has such capability and coverage. It may be noted that these priorities also may be defined on frequency level or frequency group level per RAT.
When the user equipment is operating in GERAN and has an ongoing Circuit Switched (CS) call and/or packet session, i.e. when it is in dedicated mode, dual transfer mode or packet transfer mode, there will be problems to perform measurements for all the different RATs since there are only limited measurement gaps for the user equipment. There are also limitations regarding the number of neighbouring cells that the user equipment may include in the measurements reports to the network. Therefore, even if there are neighbouring cells from several different RATs with good radio quality for the user equipment, it will not be able to measure all those cells and/or send reports for all of them to the network.
Depending on the type of service that a certain user equipment is running, it will in many cases not be preferred that the user equipment performs cell reselection to E-UTRAN or even another RAT. For example, in case the user equipment is in dedicated mode with a CS speech call ongoing, it would typically be preferable to keep the user equipment within GERAN for the duration of the call.
There has been a proposal within 3GPP TS GERAN to introduce a process to activate/deactivate measurement reporting, and possibly even the actual measurements, for E-UTRAN neighbour cells while in GERAN dedicated or dual transfer mode (DTM). The activate/deactivate command was proposed to be sent from the network to the user equipment in the ASSIGNMENT COMMAND, FREQUENCY REDEFINITION, MEASUREMENT INFORMATION, EXTENDED MEASUREMENT ORDER or HANDOVER COMMAND messages, and the corresponding DTM messages. However, even though a CS handover from GERAN to E-UTRAN would typically not be desirable, in cases where the user equipment enters areas with only E-UTRAN coverage, such a handover would still be needed. In addition, typically UTRAN would also have higher priority than GERAN for idle mode camping priority but still CS handover to UTRAN would preferably not be performed during the speech call unless needed for coverage reasons.
Also the European patent application EP 1860903 describes a method for controlling a radio cell monitoring system and the document “Drivers for inter-RAT Radio Resource Management” 3GPP DRAFT; R2-071351 provides further background.
Also, when operating in packet transfer mode, for certain types of services it will be preferred to keep the user equipment in GERAN in order to avoid an inter-RAT cell reselection during the transfer. Thus in many cases, there are differences between the priorities wanted for camping purposes when in idle mode and what is wanted for operation in dedicated, dual transfer or packet transfer mode.